A number of systems have been proposed to improve the quality of sound which is heard by a listener. One method that has been used has been to increase the number of discrete channels available for reproduction. In standard stereo systems two or more microphones are used to input the sound to a recording medium, but these inputs are mixed such that only two discrete channels are ultimately recorded. More recent systems have provided for four discrete channels, or quadraphonic sound recording and reproduction. Other systems have provided quasi-discrete means for obtaining more output audio channels from a fixed number of original channels. Such prior art systems have generated imitation four-channel stereo from the two stereo output channels, by means of complex filtering or phase modification of such input signals to generate the pseudo-quadraphonic sound. These prior art systems are also normally designed such that two speakers face the listener, and two additional speakers are positioned behind the listener, to provide a sound which has been termed three-dimensional.
Some prior art systems are known wherein a multiple number of output channels can be derived, and wherein the speakers may all be arranged in front of the listener, but such systems have involved complex methods for controling the gain to enhance stereo separation so that up to two more pseudo-channels can be derived by enhancing stereo separation for spacial expansion. See e.g. U.S. Pat. No. 3,772,479. Another system generates a third channel from stero, but this new channel outputs only monaural sound. See e.g. U.S. Pat. No. 3,491,204.